cmake: prefer system llvm library

CMakeModules/FindLLVM.cmake

@@ -0,0 +1,16 @@
+# SPDX-FileCopyrightText: 2023 Alexandre Bouvier <contact@amb.tf>
+#
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+find_package(LLVM QUIET CONFIG)
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(LLVM CONFIG_MODE)
+
+if (LLVM_FOUND AND NOT TARGET LLVM::Demangle)
+    add_library(LLVM::Demangle INTERFACE IMPORTED)
+    llvm_map_components_to_libnames(LLVM_LIBRARIES demangle)
+    target_compile_definitions(LLVM::Demangle INTERFACE ${LLVM_DEFINITIONS})
+    target_include_directories(LLVM::Demangle INTERFACE ${LLVM_INCLUDE_DIRS})
+    target_link_libraries(LLVM::Demangle INTERFACE ${LLVM_LIBRARIES})
+endif()